The present invention relates to devices for arresting shoreline erosion.
Many structures designed to prevent shoreline erosion can actually accelerate it. Groins, for instance, structures which extend out into the water from the shore and several feet above the water surface, actually force the energy of waves hitting them downwardly against the sea bottom at the bases of the groins. Sand or other loose material at the bases generally wash away, and eventually such groins will lose support and fall or sink into the sea bottom.
Conventional seawalls generally fair no better because when waves hit the seawall, they are also directed downwardly to the sea bottom. Sand washes away at the base of the seawall, so the seawall will be undermined and fall into the water, losing whatever effectiveness it may have had as an erosion control device.
Both groins and seawalls often force currents (as opposed to waves) away from shore. Normally, currents carry sand toward shore, but when groins or seawalls deflect currents away from shore, sand in the currents is carried to deeper water and the shoreline is robbed of sand which might otherwise deposit there.
There are many construction problems in building groins and seawalls. Some groins and seawalls are made of concrete or rock which must be hauled to the construction site. Large trucks filled with concrete or rock can damage sensitive beach areas which are often protected by vegetation such as dune grass which is easily uprooted by trucks, exposing the beach or dune to wind erosion. As is well known, even when a relatively small area of dune vegetation is uprooted, winds can erode away an entire dune.
Some groins or seawalls can be constructed from sand-filled fabric bags (see, e.g., Hepworth U.S. Pat. No. 3,957,098, entitled EROSION CONTROL BAG issued May 18, 1976), and the sand required can be pumped from the sea bottom nearby. Pumping sand from the sea bottom is acceptable in many situations, but in certain environmentally sensitive areas, it is not recommended or permitted.